1. Field
The present disclosure is directed to methods and apparatus for determining fractional flow reserve (FFR) which is a method of identifying the effect of an occlusion on blood flow in vasculature. Occlusions could result from a buildup of plaque, a thrombus, or any other material that prevents normal or optimal blood flow.
2. Description of Related Art
Occlusions in vasculature can be the result of the buildup of plaque, a thrombus, or any other material that prevents normal or optimal blood flow. Vascular diseases are often manifested by reduced blood flow due to atherosclerotic occlusion of vessels. For example, occlusion of the coronary arteries supplying blood to the heart muscle is a major cause of heart disease. Numerous methods are currently available for treating various lesion types, such as percutaneous transluminal angioplasty (PTCA), cutting balloon angioplasty, directional coronary atherectomy (DCA), rotational coronary atherectomy (RCA), ultrasonic breaking catheter angioplasty, transluminal extraction catheter (TEC) atherectomy, rotablator atherectomy, and excimer laser angioplasty (ELCA). Often, stents are placed within the lesion so as to prevent re-closure of the vessel (also known as recoil).
Lesion characteristics, together with vessel condition proximal and distal to the lesion and vascular bed condition, are used to determine the medically and economically optimal treatment method or combination of methods of choice. Geometry, pressure, and flow are three variables often measured in the cardiovascular system. These measurements are performed prior to, during, and after the treatment, providing diagnostic and therapeutic data. The measurement prior to the treatment allows careful treatment selection. Measurements during and after the treatment enable evaluation of the treatment efficacy.
Lesion geometry is evaluated by angiography, qualitative coronary angiography (QCA), or by intravascular ultrasound (IVUS). These measurements allow calculation of the percent diameter stenosis (angiography or QCA) or percent area stenosis (IVUS). This information is used to estimate stenosis severity, but clinicians have realized that direct physical information about pressure and flow is necessary for complete evaluation of coronary artery disease. Physiological measurements such as pressure gradient have been clinically used as an indicator for lesion severity. However, previous attempts to relate the pressure gradient across the stenosis to its functional significance have been disappointing without the use of a pharmacological agent, such as a vasodilator, that artificially increases heart rate. The decrease in the pressure gradient after PTCA has been used to assess the success of the treatment, with poor correlation. Thus, the use of a vasodilator to increase flow rate has been an important component of the foregoing measurement calculations.
Other parameters have been defined and proven more effective as indicators for lesion severity. The coronary flow velocity reserve (CFVR) is defined as the ratio of hyperemic to baseline flow velocity. The fractional flow reserve (FFR) is defined as the ratio of distal (to stenosis) pressure (Pd) to aortic pressure (Pa) during hyperemia. Hyperemic conditions are obtained by administration of vasodilators (e.g., papaverine, adenosine). Clinical studies have demonstrated that, in most cases, lesions with CFVR<2 must be treated using one of the previously mentioned methods, whereas for patients with CFVR>2, angioplasty may be avoided. Similarly, in many cases interventions, such as angioplasty, may be avoided if FFR>0.75. Coronary flow occurs essentially during diastole while systolic contribution to total coronary flow is smaller. A notable difference between diastolic to systolic velocity ratio (DSVR) was observed between normal and stenotic arteries.
The FFR and CFVR are independent but complementary indicators. The first characterize the specific lesion whereas the second is a more global parameter, characterizing the lesioned vessel (lesion and distal bed). Clinical studies (Di Mario et al., Catherization and Cardiac Diagnosis 38, 189-201, 1996) show that for approximately 75% of the patients, CFVR and FFR lead to the same conclusion regarding the lesion significance. At the same time, for 25% of the patients, the conclusions regarding lesion significance were different. This means that simultaneous determination of coronary flow reserve and fractional flow reserve is important and gives the clinician the additional and more complete information regarding the lesion severity.
Technical progress has been made recently with respect to pressure and velocity monitoring guide wires. For example, 0.014″ PressureWire® (Radi Medical System, Uppsala, Sweden) is now available for intracoronary pressure measurements. Another maker of pressure wires is Volcano Corporation and sold under the trade name FloWire®. Additionally, these measurements may be performed using diagnostic low profile catheters, Millar pressure transducer catheters (available by Millar Instruments, Inc., Houston, Tex., U.S.A.), or any other intravascular pressure equipment.
A 0.014″ doppler flow wire (Cardiometrics Inc., Mountain View, Calif.) is also available for intracoronary velocity measurements. These wires may be advanced into distal parts of the coronary tree without significantly impeding the flow. Simultaneous measurements of FFR and CFVR require the use of both wires and/or a wire with multiple sensors. Such a procedure is complicated, expensive, and used only for research purposes. Therefore, clinicians use either velocity measurements to calculate coronary flow velocity reserve (CFVR) or pressure measurements to calculate fractional flow reserve (FFR). Furthermore, the flow wire is sensitive to the location of the tip within the vessel cross section. The wire tip will measure accurately if located along the longitudinal axis. However, significant errors will appear once the wire is within the boundary layer. Therefore, manipulating the flow wire requires high expertise and a lot of experience. Fortunately, these limitations are not relevant to the pressure wire measurements, yielding accurate data with simple handling.
Relevant United States patents and publications in the field of methods and apparatus for determining fractional flow reserve (FFR) in blood vessels include U.S. Pat. No. 6,089,103 to Smith; U.S. Pat. No. 6,354,999 to Dgany et al.; U.S. Pat. No. 6,471,656 to Shalman et al.; U.S. Pat. No. 6,565,514 to Svanerudh et al.; U.S. Pat. No. 6,615,667 to Smith; U.S. Pat. No. 6,672,172 to Tulkki et al.; U.S. Pat. No. 6,754,608 to Svanerudh et al.; U.S. Pat. No. 7,454,244 to Kassab et al.; U.S. Pat. No. 7,775,988 to Pijls, each incorporated herein by reference in their entirety for this purpose. U.S. Patent Application Publication No. 2009/0234231 to Knight et al. and International Publication No. WO 2010/033971 to Kassab likewise are directed to known methods and apparatus for determining fractional flow reserve (FFR) in a blood vessel and are incorporated herein by reference in their entirety for this purpose.